Local Relief Model (LRM) Toolbox for ArcGIS

2014 David Novk

Local Relief Model (LRM) Toolbox for ArcGIS

Guide for application of Local Relief Model in ESRI ArcGIS 10.x software using Spatial Analyst and 3D Analystextensions. Those two extensions have to be licensed to make the utility work properly.UPDATE 2014-10-7: The toolbox was updated to work with ArcGIS 10.2. In old version python script wasmessed up and application was unstable. Due to the changes it is needed to set the cellsize of input DEM atthe start of the process.Download link to get the toolbox (there are both old and new version):https://drive.google.com/folderview?id=0Bxw2O48YquY4eG5aTTloSTNWdjg&usp=sharing

Local Relief Model (LRM) Toolbox for ArcGIS

IntroductionVisualisation of elevation data collected by aerial laser scanning (ALS/LiDAR) for archaeological use is widelydiscussed topic. Every method has its advantages and disadvantages in usability, efficiency, timeconsumption etc. It is obvious that simple shaded relief model (hillshade) is not sufficient for archaeologicalresearch and there are much better ways of visualisation. 1 According to J. Bofinger and R. Hess (2011),2local relief model (LRM) is one of the most useful and efficient way to visualise elevation data stored inraster as digital elevation model (DEM). The basic idea behind the method is filtering out terrain surfaceleaving just archaeological features and their relative elevation above or below the terrain. The simplestway to do so is to generalize DEM by low pass filter and subtract it from original DEM as it is suggested byO. Davis in his simple guide (Davis 2012, 15-16). However, this is only the first step of the process and it willcreate a very biased result, called difference model. To create fully acceptable LRM (Bofinger Hess 2011,166-167) it is needed to:1) Apply low pass filter3 to the original DEM -> smoothed DEM (Focal Statistics function)2) Subtract smoothed DEM from the original DEM -> Difference model (Minus function)3) Calculate the zero meters contours for the Difference model -> set of the break lines between thepositive and negative features (convex and concave features; Contour function)4) Extract the real elevation of the cells from the original DEM, which intersects with the break lines > simplified elevation raster (Extract by Mask function)5) Convert the simplified elevation raster to the point features (only nonzero cells) -> elevationpoints (Raster to Point function)6) Create the simplified surface DEM from the elevation points -> computed digital terrain model(DTM; Create TIN and TIN to Raster functions).7) Subtract the DTM from the original DEM -> LRM (Minus function)This more complex process leads to creation of flat terrain LRM, where values express differencebetween expected terrain elevation (computed DTM) and real elevation of DEM. It is extremely usefulwhen we using data from aerial laser scanning (ALS/LiDAR) for identification of potential archaeologicalfeatures as it is designed exactly for this purpose.Application of this workflow is not complicated, but for common user it may be difficult to apply all thefunctions referred above correctly. Creating the workflow for ArcGIS is the time consuming activity andthere is no another user-friendly tool applicable with ArcGIS, which would make this task easier. For thatreason I have created simple toolbox for ArcGIS with pre-prepared tool, which create LRM from any givenDEM raster. All used functions and tools are licensed by ESRI ArcGIS.

For more about DEM visualization topic and comparison of methods see Doneus 2013.Described workflow is just application of Bofingers and Hesss article and the author does not intend to usurp anybasic ideas of the method as his own.3Low pass filter is method to smooth the DEM. It goes through the data and calculates cell value as average ofsurrounding cells. The radius of which the mean value is computed is defined as polygon of exact dimensions. Widerpolygon (or processing window) leads to smoother result and thinner polygon keeps more of the data. Setting of thisvalue is crucial to the shape of any result.2

Local Relief Model (LRM) Toolbox for ArcGIS

ESRI ArcGIS Desktop 10.0

How to use it1) Install the LRM toolbox by adding it to your ArcToolbox.2) Run the LRM tool.3) Set:

a. Input DEMb. Cellsize of input DEM (you have to keep prefix 'CELLSIZE' in the field)c. Low pass window shape and extent (most recommended settings are set as default)d. Output foldere. Click OK.4) After all tasks are done (it may take several minutes depending on the extent and detail of inputDEM, set input values and your hardware), utility will create DTM and LRM raster dataset at yourspecified output location (all intermediate data except DTM will be removed automatically). DTMraster layer will be added to your project display automatically4 and LRM raster layer have to beadded to display manually.5) Add LRM to your project and set desired symbology in layer properties (for example stretched; bluecolours for concave features; red colours for convex features; stretch type set to standarddeviation; n = 1).

IMPORTANT NOTE: If you want keep generated DTM raster, you have to move it to another location or rename it orset different output folder for next run of the utility. If you would not do that, DTM will be overwritten.

Local Relief Model (LRM) Toolbox for ArcGIS

Terms of useBy downloading or using LRM Toolbox, you agree to the following terms and conditions:Toolbox is open and free to use and modify by any user. Use, copy, share and do whatever you wish withthis software only at your own risk. The author takes no responsibility of possible damage or otherproblems with your software, hardware or data caused by using the LRM Toolbox.